Abstract—Double-gate field-emission characteristics of metallicfield-emitter array (FEA) cathodes fabricated by molding withstacked collimation gate electrodes with planar end plane arereported. Collimation of the field emission electron beam withminimal reduction of the emission current was demonstrated whena negative bias was applied to the collimation gate, whereas whenthe two electrodes were at the same potential, the emissioncharacteristic of the double-gate device was same as that of thesingle-gate device that shows emission current of ~1mA from40x40 tip arrays. The results indicate that the device structure ofthe fabricated double-gate FEAs is promising for high-brilliancecathode applications.

DOUBLE-gate field-emitter-array(FEA)cathodeshaving a collimation gate electrodeGcstacked on top of the electron extractiongate electrodeGex

have been studied in the past for the purpose of eliminating pixel-to-pixel crosstalk in field-emitter displays[1]-[3], for field-ionizer applications [4], and for electron-beam lithography applications[5]–[7]. FEAs have also been studied asthe cathode for a compact free electron laser with sub-nanometer wavelength [8]: FEAs can be competitive with thestate-of-the-art photocathode [9], [10] when the angular spread

of individual beams is reduced below ~1° while keeping theaverage current density

andemitters as reported in the literatures [1-7].However, since thenegativeVc

can reduce

the electric fieldFapx

at the tip-apexandthe emission current, the optimization of the device structure minimizing the influence ofVconFapx

is crucial. Forthehigh-brilliance applications

in an acceleration gradient ofthe order of 100 MV/m,device structures with minimal protrusion arepreferred to prevent the parasitic breakdown.Our previous approach [12] based onthemolded

FEAs having the stackeddouble-gate electrodes showed successful operation of the device but the emission current decreased substantially by negativeVc.In thisLetter, wetherefore exploretheimprovedfield-emission current-voltage characteristics of double-gate FEAs in amodifiedgate aperture geometry.

II.

SAMPLE ANDEXPERIMENT

We fabricated single-gate FEA devices, SG1 and SG2, and a double-gate FEA DG having 4040 tip array. SG1 was fabricatedusing a FEA wafer with apex diameteraapxof ~10 nm. SG2 and DG were fabricated using a FEA wafer withaapxof ~20 nm. Inaddition, a 44 tip double-gate FEA was fabricated together with DG. The FEA wafers were fabricated by the molding method[12]-[15] supported by0.4 mm-thickelectro-platednickel. The emitters have1.5 µm-square base sizeand

~1.2 µm-height,aligned with5 µm-pitchin the arrays.Gexlayer was 0.5µm-thick Mo film separated from the arrays by 1.2 µm-thick SiO2filmdeposited by plasma-enhanced chemical vapor deposition. For thedouble-gate FEAs,Gclayer of 0.5µm-thick Mo was addedon

Manuscript received May 6, 2010. This work was supported in part by the SwissFEL project, Paul Scherrer Institute and in part by the Swiss National ScienceFoundation No.200021-125084.

Fig. 1. (a) Scanning electron microscope image of thedouble-gate FEAcathode with 4040 tips. The emitters are aligned with 5 µm-pitch. (b) Highresolution image of one of the emitters from (a). The apex of the molybdenumemitter can be seen as the bright spot inside the extraction gate aperture.

2

top of the extraction gateseparated by1.2 µm-thick SiON.Thediameter ofGexapertures of SG1 and SG2 were equal to 2.3±0.1µm. The aperture diameters of DG were equal to 1.2±0.1µm forGexand 3.5±0.1µm forGc, respectively.The detailsof thefabrication procedure were described elsewhere[12].

The field-emission characteristics were measured in the setupsshown in Fig. 3 (a) and (b).The field-emission microscopy

Vcis five orders of magnitude weaker than that toVge.Fig. 2(b) showsIa, the currentIeminjected to the emitter substrate andthe currentIcthroughGc. We observe tendencies thatIcincreasesfaster thanIaandIemfor positiveVcand a slight increase of Icwith the decrease ofVcforVcbelow-20 V. The former can beascribed to the increased capture of the field emission electronsbyGcwhile the latter can be ascribed a field emission from theGexedges toGcas observed in Ref. 16]. Neverthless,

Icas wellas the difference betweenIemandIaare less than 5 % ofIaforVc

below 0 V; the capture of the field-emission electrons byGex

andGcis minimal and that the gate leak currents are small.

The observedemission currentcharacteristic fits well to theequation,Ia=AVnexp(-B/V), withnequal to 2 [17] and with thetotal effective bias voltageVequal to (Vge+Vc), where

is thecontribution ofVcto the apex field. From the result of Fig. 2, weevaluated

to be equal to (0.17±0.014). The evaluation errorrepresents the bound that the rms spread of the quantity ln(Ia/V2)is below 4% whenVis equal to 60 V forVcbetween-70 and +70V. The observed value of

was derived by assuming thatFapxis proportional to [Vge/Dex+(Vc+Vge)/Dc]. We alsonote that the previously reporteddouble-gate device [12], that was fabricated from the sameemitter array as DG and exhibited a reduction ofIaby a factor of 103forVcof-70 V, had a factor of ~3 largerDex/Dc

ratio and

–value than the present device. This is consistent with the above analysis.

Iaof the single-gate devices reach ~1 mA atVgeof 130-150 V. The maximumIaofDG was somewhat lower due to the premature failure of the device but itsIa-Vgecharacteristic is same as that of SG2within ~5 VofVge. Thisshowsthe uniformity of the single-and double-gate fabrication processes over the 40

40 tips.

Finally, to study the effect ofVcon the electron beam collimation, we measured the beam profile in low current regime, Fig. 4,using the double-gate device having 44emitters. Similarly to the large array emitter, the decrease ofIemfor the 44 emitterarray was 20% whenVcwas decreased from 0 to-70 V, Fig.4 (d). Fig. 4 (b) shows that whenVgewas fixed at 86 V, the beamexhibited the emission angle

of (20±3)° forVclarger than-30 V.

was evaluated from the full-width at the half maximumsize of the intensity distribution of the phosphor screen image and the screen-FEA distanceD.This value is consistent with theprevious observation for single-gate Spindt-type FEAs [18], [19]. WhenVcwas further decreased to-62 V,

was decreased to(2.3±0.4)° in one direction. The asymmetry and distortion of the collimated beam shape

should be improved by careful design ofthe electrode shape [20] and by elimination of the parasitic field due to the screen assembly, the extraction gate, the electrical

Fig. 3. Schematic diagram of the measurement setup of single-gate FEAs (a)and double-gateFEA (b). The FEAs and the anode (separated by 10 mm)were mounted in the vacuum chamber (background pressure of ~10-9mbar),represented by the enclosed area. (c) Anode currentIavs extractiongate-emitter biasVgefor two single-gate FEAs, SG1, SG2, andand thedouble-gate FEA, DG with the biasVcat the collimation gateGcfixed at 0 V.All the devices have 4040 emitters. SG1 was fabricated using an emitterarray with emitter apex diameter of ~10 nm. SG2 and DG were fabricatedusing arrays with emitterapex diameter of ~20 nm.

contact assembly of the FEA mount, and the aperture shapes inthe future experiment. Detailed analysis of the observedcollimation characteristic and its comparison with theory willbe described elsewhere [21].

In summary, we showed that by engineering the aperturesizes it is possible to collimate the field emission electron beamwhile minimizing the emission current reduction in double-gateFEAs with stacked

Gcwith planar end plane. Furtheroptimization of the device structures such as the gate electrodethicknesses [7], the gate insulator thicknesses, and the gateaperture sizes are the next subjects of the research.